Stability of emulsions

                           Stability of emulsions

•A stable emulsion may be defined as a system in which the globules retain their initial character and remain uniformly distributed throughout the continuous phase.
•The three major phenomena associated with physical stability are
(1) The upward or downward movement of dispersed droplets relative to the continuous phase, termed Creaming or sedimentation, respectively
(2) The aggregation and possible coalescence of the dispersed droplets to reform the separate, bulk phases, termed cracking,
(3) Inversion, in which an o/w emulsion invert to become a     w/o emulsion, and vice versa.
1-Creaming:

nCreaming is the separation of an emulsion into two regions one of which is richer in the disperse phase than the other.

 E.g., creaming of milk when fat globules slowly rise to the top, of the product.

nThis is not a serious instability problem as a uniform dispersion can be obtained simply by shaking the emulsion,
nCreaming is undesirable from a pharmaceutical point of view because;
•Creamed emulsion is inelegant in appearance and provides the possibility of inaccurate dosage
•Increases coalescence possibility since the globules are close together in the cream.
•These factors are indicated by Stokes' law as follows:

V = d2(p –po) g / 18 ηo

where V is the velocity of creaming, d the globule diameter, P-Pothe densities of disperse phase and dispersion medium respectively and ηothe viscosity of the dispersion medium.

•A consideration of this equation shows that the rate of creaming will be decreased:

1-by reduction in the globule size.

2-a decrease in the density difference between the two phases, and

3-an increase in the viscosity of the continuous phase.

•This may ,be achieved by

nReduce the globule size
nIncreasing the viscosity of the continuous phase by the use of thickening agents such as Tragacanth or methylcellulose.
It is seldom possible to satisfactorily adjust the densities of the two phases.
2-Cracking or breaking( Coalescence):

Cracking is the coalescence of dispersed globules and separation of the disperse phase as a separate layer. It is an irreversible process and redispersion cannot be achieved by shaking.

•Factors that cause an emulsion to crack are:

1-The addition of chemical that is incompatible with the emulsifying agent, thus destroying its emulsifying ability.

Examples include surface-active agents of opposite ionic charge, e.g. the addition of cetrimide (cationic) to an emulsion stabilized with sodium oleate (anionic).

2-Bacterial growth: protein materials and non-ionic surface-active agents are excellent media for bacterial growth.
3-Temperature change:

nheating above 70°C destroys most emulsions.
nprotein emulsifying agents may be denaturated 
nthe solubility of non-ionic emulsifying agent change with a rise in temperature
nFreezing will also crack an emulsion; this may be due to the ice formed disrupting the interfacial film around the droplets.
3-Phase inversion:
•Soaps of divalent metals produce w/o emulsions while soaps of monovalent metals (and ammonium soaps) produce o/w emulsions. 
•An o/w emulsion stabilized with sodium stearate can be inverted to the w/o type by adding calcium chloride to form calcium stearate (phase inversion).